1. Field of the Invention
The invention relates to a solenoid valve.
2. Description of the Prior Art
A conventional solenoid valve, particularly for a hydraulic unit which is used for instance in an anti-lock brake system (ABS) or a traction control system (TC system) or an electronic stability program system (ESP system), is shown in FIG. 1. As can be seen from FIG. 1, a conventional solenoid valve 1, which is closed when without current and which is embodied as an on-off valve, includes a valve cartridge 2 and a magnet assembly 13. The valve cartridge 2 includes a pole core 3, a valve insert 5 connected to the pole core 3 and embodied as a sleeve, an armature 4 guided axially movably inside the valve insert 5 between a closed position and an open position, which armature is coupled with a closing element 8, and a valve body 9, connected to the valve insert 5, with a main valve seat 10 that is disposed between at least one first flow opening 11 and a second flow opening 12. The valve body 9 is likewise embodied as a sleeve. The magnet assembly 13 includes a winding holder 13.1, a housing jacket 13.2, a coil winding 13.3 with electrical terminals 13.5, and a cover disk 13.4. The axially movable armature 4, when current is supplied to the magnet assembly 13, or in other words when current is applied to the coil winding via the electrical terminals 13.5, is moved counter to the pole core 3 by a generated magnetic force Fmagnetic inside the valve insert 5 counter to a spring force Fspring of a restoring spring 6 and counter to a fluid force Fhydraulic, in order to lift the closing element 8 out of the main valve seat 10 and to enable a fluid flow between the at least one first flow opening 11 and the second flow opening 12. The maximum possible stroke of the armature 4 and of the closing element 8 is specified by the air gap 7 between the pole core 3 and the armature 4. In order to press the closing element 8 sealingly into the main valve seat 10 and to interrupt the fluid flow between the at least one first flow opening 11 and the second flow opening 12, the axially movable armature 4 with the closing element 8 is moved away from the pole core 3 in the direction of the main valve seat 10 by the spring force Fspring of the restoring spring 6 and by the fluid force Fhydraulic inside the valve insert 5. The pole core 3 is connected in fluid-tight fashion to the valve insert 5, for instance by means of a weld seam. Moreover, the solenoid valve 1 shown is calked via a calking flange 16 to a fluid unit block 15. The action directions of the spring force Fspring, fluid force Fhydraulic, and magnetic force Fmagnetic are indicated by arrows in FIG. 1.
Until now, outlet valves in the ABS/ESP system have been embodied purely as on-off valves, which can be operated in quasi-stationary fashion as only fully open or fully closed. The outlet valve, as a noncontinuous on-off valve, typically experiences a flow through it in a stroke-closing direction. This is done in the context in which high wheel pressures reinforce the valve tightness. Thus only slight initial spring tensions suffice, which permits a lower total force level and thus allows fast valve reactions.
Compared to on-off valves, continuous valves have the advantage that by adjustment of partial strokes between the fully closed and the fully open position, arbitrarily adjustable intermediate positions can be established, and thus arbitrary flow cross sections can be opened, and arbitrary flow quantities through the valve can be established. For the valves of an ABS/ESP system, this means for instance that the buildup and reduction gradients of a wheel pressure can be adjusted variably, and as a result the meterability of the wheel pressure can be improved and the noise-generating pressure fluctuations can be reduced. A continuous valve through which the flow is in the opening direction, however, requires a high initial spring tension for sealing off a requisite wheel pressure, such as a locking pressure level. However, as a result the total force level also becomes quite high, which when current is supplied to the magnet assembly leads to high current intensities and is disadvantageous with regard to the valve reaction times, current adjustment precision, and the thermal performance.